Initial support for multi-target tree. (#8616)

* Implement multi-target for hist.

- Add new hist tree builder.
- Move data fetchers for tests.
- Dispatch function calls in gbm base on the tree type.

src/common/quantile.h

@@ -352,19 +352,6 @@ struct WQSummary {
       prev_rmax = data[i].rmax;
     }
   }
-  // check consistency of the summary
-  inline bool Check(const char *msg) const {
-    const float tol = 10.0f;
-    for (size_t i = 0; i < this->size; ++i) {
-      if (data[i].rmin + data[i].wmin > data[i].rmax + tol ||
-          data[i].rmin < -1e-6f || data[i].rmax < -1e-6f) {
-        LOG(INFO) << "---------- WQSummary::Check did not pass ----------";
-        this->Print();
-        return false;
-      }
-    }
-    return true;
-  }
 };
 
 /*! \brief try to do efficient pruning */